1. Field of the Invention
The invention is directed to a method of synchronizing a receiver with a received ambiguous signal having a known number of at least two peaks.
2. Discussion of Background Information
In the frame of Radio Navigation Signals, new modulated shape signals have been designed to fulfil new requirements, such as high accuracy, good spectral separation between Signals. Despite their good quality in terms of performance, these new modulations have also introduced new limitations like the ambiguity constraint of the multiple peaks in the auto-correlation function. A synchronization on a wrong peak of the auto-correlation function (ACF) would cause a code delay error, which would itself generate an error in the positioning. The invention will detect any synchronization on a wrong peak and offer the possibility to correct it.
Many techniques have already been proposed to solve this problem:
A first known technique is the Bump Jumping algorithm. In this technique the receiver evaluates the absolute value of the amplitude of two samples of the correlation function which are spaced by N/(2M) chip with respect to the prompt (central) correlator (the N/(2M) values apply for the case of a binary offset carrier BOC(M,N) without filtering). Then the receiver uses the amplitude of these two side peaks, as well as the amplitude of the prompt channel, to verify if the working point of the corresponding tracking loop is effectively well aligned on the central peak of the correlation function and not shifted of one or more sub-carrier symbols (a sub-carrier symbol is equal to N/(2M) chip). In case of an incompatibility between the observed amplitudes and the expected ones, the receiver applies an offset to the replica for correction and to achieve again a perfect alignment. This verification and correction (if needed) can be performed either after each integration period or at a lower rate.
This algorithm shows some limitations as soon as the transmitted signal contains some distortions or when multipath degrade the shape of the auto-correlation function. Indeed, in these cases, the amplitude of the corresponding side peaks of the correlation function can become very close to the amplitude of the central peak (especially true for large M/N ratio and/or relatively low Signal-to-Noise ratio). In that case, a biased comparison of the three correlation peaks would lead to an incorrect decision and to an unnecessary application of an offset to the replica. This would create a large error in the range estimation (several meters). Therefore, the Bump Jumping is restricted to the monitoring of two first side peaks of the ACF and not the complete ACF.
A second known technique is the Double Estimation Technique (DET). The DET consists in implementing an additional tracking loop beside the conventional Delay Lock Loop (DLL) and the Phase Lock Loop (PLL). This loop, called Sub-Carrier Lock Loop (SLL) tracks the sub-carrier embedded in the BOC signal. The sub-Carrier frequency is equal to the chip rate frequency divided by (2M/N for a BOC(M,N) signal). Once the corresponding sub-carrier replica is aligned with the sub-carrier embedded in the received signal, the DET removes it by simple multiplication; this can be considered as a sub-carrier wipe-off. The resulting output of this operation is a signal with a waveform very close to a binary phase shift keying BPSK(N). In that case the DLL will use a BPSK(N) replica to track the corresponding BPSK signal. The favorable property of the corresponding BPSK correlation is that it is unambiguous. However the corresponding peak is not that sharp. Therefore the corresponding tracking performances are worse than those of the SLL which uses a Sub-Carrier Correlation function whose peaks are much narrower (for large M/N ratio). On the other side the correlation function for the sub-carrier replica is multimodal with 2M/N possible stable locking points, and is therefore ambiguous. Hence a complementary exists between the correlation function of the DLL (lower tracking performance but unambiguous) and the SLL (larger tracking performance but ambiguous). The DET then uses the unambiguous correlation function (from the DLL) to correct any offset of the working point of the SLL. The final DET output (range estimation) consequently benefits of the high performances of the SLL and on the ambiguity correction of the DLL.
The DET has been shown to maintain reliable and robust acquisition and tracking in high noise conditions, but the technique may not be as efficient in a real environment (multipath) or in presence of a distorted signal. Indeed both previous effects lead to an asymmetrical BPSK correlation function used by the DLL. If such asymmetries are too large then the correction brought by the unambiguous (and distorted) correlation function for the correction of the SLL might be wrong. This is the possible drawback of the DET algorithm.
A third known technique is the unambiguous tracking structure. This method is very close to the DET algorithm previously described. It consists in generating a correlation function having a single peak which is wider than the central peak of the ambiguous correlation function. A Sub-carrier tracking loop which provides the tracking performance is also implemented. In fact alterative discriminators for the DLL which process this unambiguous correlation function are applied and are different to the discriminator used for the DET. Furthermore, the module which performs the correction of the range estimation provided by the SLL, based on the output of the new DLL is also different. It is possible to determine the threshold for a correction brought to the SLL ranging output, as a function of a specified probability of detection and/or false alarm.
If an ambiguity has been detected, this method has the disadvantage to track on the signal without sub-carrier, which is much more sensitive to multipath and therefore degrades the performances.
A fourth known technique is the Enhanced Sidelobes Cancellation Method. The receiver uses a replica whose waveform is different (not matched) from the waveform of the received signal (BOC(M,N)). The corresponding correlation function shows side-peaks whose amplitude is significantly lower than the side peaks of the ambiguous correlation function that would be derived with the matched (BOC(M,N)) replica. This new correlation function therefore provides a reduced probability of locking onto such side peaks (the ambiguity is thus resolved) and improves the detection performance and correction performance of algorithms like the Bump Jump algorithm.
This method solves the ambiguity, but a correlation loss appears due to the use of a signal replica shape different from the received one.
A fifth known technique is the Binary offset carrier M-code envelope detector that is described in U.S. Pat. No. 7,555,033. This method proposes to create a correlation function presenting a single peak in a different manner than the DET and the “Unambiguous Tracking Structures”. This correlation peak is not generated by wiping-off the sub-carrier from the received signal, because this sub-carrier has been successfully tracked by the mean of an SLL. Here the correlation peak is created by combining two correlation outputs: one which is generated with the in-phase and another with the quadrature phase BOC sub-carrier. The non-coherent combination of both correlation outputs provides hence a single and unambiguous peak. This method is also called Sub-Carrier Cancellation.
This method solves the ambiguity. However, its tracking performances are lower than those obtained with the original BOC signal because on one side the correlation peak is wider (smaller time resolution) and on the other side the noise contribution is increased due to the combination of several correlation functions.
In general it can be stated that except for the Bump Jump method all alternative methods try to create an unambiguous correlation peak (Side peak Cancellation, DET, Unambiguous tracking structure and Sub-Carrier cancellation). This one is either used directly for the provision of the range estimation (in case of the Sub-Carrier cancellation method) or used to correct any possible wrong tracking point of the Sub-carrier tracking loop (DET, Unambiguous tracking structure).